In the field of motor vehicle transmissions, it is known to provide torque transmission devices with torsional vibration dampers allowing absorption and damping of the vibrations and acyclisms generated by an internal combustion engine.
The vibration dampers comprise an input element and an output element which are rotatable around a common axis of rotation, and elastic damping means for transmitting the torque and damping the rotational acyclisms between the input element and the output element.
Such vibration dampers are used in particular on dual mass flywheels (DVA), clutch friction plates in the case of manual or semiautomatic transmission, or lock-up clutches fitted to hydraulic coupling devices in the case of automatic transmission.
Documents FR 2894006, FR 2913256 and FR 2922620 illustrate vibration dampers fitted respectively to a dual mass flywheel, a clutch friction plate and a lock-up clutch. The elastic damping means used on these vibration dampers are helicoidal springs with circumferential effect, the ends of which come to rest firstly against stops integral with the input elements and secondly against stops integral with the output elements. Thus any rotation of said elements relative to each other causes a compression of the damper springs in one direction or the other, and said compression exerts a return force able to return said elements to a relative angular rest position. The helicoidal springs may be straight or curved.
The stiffness of such a damper is determined as a function of the number of helicoidal springs used therein, the intrinsic stiffness of the springs and the installation diameter of the springs. The choice of stiffness of such vibration dampers results from a compromise between the acyclism filtration efficacy, which increases as the stiffness diminishes, and the capacity to transmit the maximum engine torque without the coils of the springs coming to rest against each other, which requires a sufficient stiffness.
In order to improve the filtration performance for low-torque vibrations, it is known to provide vibration dampers for which the characteristic curve of the torque transmitted as a function of the angular travel has several gradients. Thus at low torque, the stiffness of the damper is less, whereas when approaching the maximum engine torque to be transmitted, the stiffness of the vibration damper is greater. Such a vibration damper is described in particular in document EP 2157336. However, the stiffness change zones cause discontinuities and shocks which are harmful to the quality of the acyclism damping.
Also, as the helicoidal springs are arranged circumferentially, they are extremely susceptible to centrifugal force. Also, the input and/or output elements must be equipped with means allowing the springs to be retained radially and hence prevent their ejection. However these radial retention means introduce parasitic friction, which affects the damping function by blocking the springs when the rotation speed is too high. Attempts have been made to reduce the effect of the parasitic friction by providing complex geometries, surface treatment, or by the introduction of grease. However these measures render production of the vibration dampers more complex and are not therefore totally satisfactory.
Finally, since the space available for the helicoidal springs is limited, the angular travel between the input and output of the vibration damper is also limited, and the helicoidal springs must have a sufficient stiffness to allow transmission of the maximum engine torque.
Thus vibration dampers with helicoidal springs are not totally satisfactory.